On Fri, 30 Aug 2019 15:45:13 +0000 Parav Pandit <parav@xxxxxxxxxxxx> wrote: > > > > > > This detour via the local variable looks weird to me. Can you > > > > > > either create the alias directly in the mdev (would need to > > > > > > happen later in the function, but I'm not sure why you generate > > > > > > the alias before checking for duplicates anyway), or do an explicit copy? > > > > > Alias duplicate check is done after generating it, because > > > > > duplicate alias are > > > > not allowed. > > > > > The probability of collision is rare. > > > > > So it is speculatively generated without hold the lock, because > > > > > there is no > > > > need to hold the lock. > > > > > It is compared along with guid while mutex lock is held in single loop. > > > > > And if it is duplicate, there is no need to allocate mdev. > > > > > > > > > > It will be sub optimal to run through the mdev list 2nd time after > > > > > mdev > > > > creation and after generating alias for duplicate check. > > > > > > > > Ok, but what about copying it? I find this "set local variable to > > > > NULL after ownership is transferred" pattern a bit unintuitive. > > > > Copying it to the mdev (and then unconditionally freeing it) looks more > > obvious to me. > > > Its not unconditionally freed. > > > > That's not what I have been saying :( > > > Ah I see. You want to allocate alias memory twice; once inside mdev device and another one in _create() function. > _create() one you want to free unconditionally. > > Well, passing pointer is fine. It's not that it doesn't work, but it feels fragile due to its non-obviousness. > mdev_register_device() has similar little tricky pattern that makes parent = NULL on __find_parent_device() finds duplicate one. I don't think that the two are comparable. > > Ownership transfer is more straight forward code. I have to disagree here. > > It is similar to device_initialize(), device init sequence code, where once device_initialize is done, freeing the device memory will be left to the put_device(), we don't call kfree() on mdev device. This does not really look similar to me: devices are refcounted structures, while strings aren't; you transfer a local pointer to a refcounted structure and then discard the local reference.
